High temperature diffusion furnaces are well known to the semiconductor industry. Heat treatment in high temperature diffusion furnaces is a part of the manufacturing process for silicon wafers whereby, for example, doping elements such as boron can be introduced into the molecular structure of the semiconductor material. Heating cycles for the furnaces must be controlled accurately with respect to time and temperature. There is also a requirement that the diffusion furnace be made durable enough to withstand repeated heating and cooling cycles. Further, for purposes of the manufacturing processes, it is important that the diffusion furnace quickly reach the desired temperature, maintain the temperature for a preselected period of time and then quickly reduce the temperature to the desired level.
Furnace Design:
All of the above requirements dictate that the design of the diffusion furnace have the goals of (1) reducing the mass of the diffusion furnace and (2) exposing the heating elements as much as possible so that the maximum desired temperatures are achievable and so that the mass of the furnace does not unduly effect efficient operation. Further, it is important that the mass of the furnace be sufficient to insulate the rest of the environment. Additionally, the heating elements should be adequately positioned and restrained so that they do not grow as described hereinbelow and so that the heating elements do not fail, requiring costly replacement and resulting in damage to semiconductor products.
In actual practice the diffusion furnaces used in the semiconductor industry are substantially cylindrical in shape. All diffusion furnaces are equipped with a process tube in which the silicon wafers are processed. The process tube is fabricated of quartz, polysilicon, silicon carbide or ceramic. The processing tube 21 is inserted into the diffusion furnace as shown in FIG. 1
The silicon wafers to be heat treated are mounted into boats, fabricated of quartz, polysilicon, silicon carbide or ceramic, and loaded either manually or automatically into the process tube.
The existing diffusion furnaces 20 include an outer metallic housing 22, usually comprised of stainless steel or aluminum and inner layers 24 of insulating materials such as a ceramic fiber. Several helical heating elements 26, 28 and 30 are secured together to form one continuous element with the middle heating element 28 operated at the optimal temperature and the end heating elements 26, 30 operated to a temperature sufficient to overcome losses out the end of the furnace and to preheat any gases being introduced into the furnace. The heating element is generally a helically coiled resistance wire made of a chrome-aluminum-iron alloy The wire is generally heavy gauge 0.289 inches to 0.375 inches in diameter) for longer heating element life at an elevated temperature.
The maximum permissible operating temperature for the heating element alloy is 1400.degree. C. Since a temperature differential exists between the heating element and the inside of the process tube, diffusion furnaces are normally operated at a maximum operating process chamber temperature of 1300.degree. C.
Heating Element Spacers:
Ceramic spacers, such as spacers 32 and 34 as shown in FIGS. 2A and B, 3A and B and 4 are used to separate and hold in place the individual coils, turns or loops of the helical heating element. Maintenance of the correct separation between each coil or turn is critical to the operation of the furnace which normally require a maximum temperature differential of no more than .+-.1/2.degree. C. along the entire length of the center zone. Electrical shorting between turns and interference with uniform heat distribution can result if the gaps between the turns or loops changes.
As shown in FIGS. 2A and B, a first type of spacer 32 is known as a comb type spacer. This comb type spacer defines a plurality of recesses 38, each of which can receive a turn or individual coil of the helical heating element. Multiple spacers 32 are butted together along the length of the furnace 20 in order to support the entire length of the helical heating element. Further, as can be seen in FIG. 5, the ceramic spacers 32 are positioned circumferentially about the internal diameter of the diffusion furnace 20 in order to support the coil circumferentially.
FIGS. 3A and B depicts an individual type spacer 34 which is also used with helical heating elements. As can be seen in FIG. 4, where multiple spacers 34 are held together in order to hold the helical heating element in place, each individual spacer 34 defines first and second wire retention recesses 40, 42. Each of these recesses defines half of a cavity for retaining a loop of wire of the heating element. Thus, as can be seen in FIG. 4, loop 44 is retained between the wire retention recess 40 and the wire retention recess 42 of two adjacent individual spacers 34. These spacers 34 abut against each other.
Generally the insulation 24 is comprised of a ceramic fiber insulating material having 50% alumina and 50% silica. This insulating material is applied to the exterior of the heating element after the turns are positioned within the spacers. The insulation is applied either as a wet or dry blanket wrapped around the heating element or is vacuum formed over the element. After the insulation has dried, it keeps each spacer and in combination with the spacer, each turn or coil of the helical heating element properly aligned.
It is known that after furnaces are placed in service and generally after eight to ten hours of operation at a minimum temperature of about 1000.degree. C., that an aluminum oxide coating forms over the surface of the heating elements. The aluminum oxide layer or coating is beneficial in that it retards thermal elongation of the heating element at high temperatures, prevents contaminants from collecting on the surface of the heating elements and protects the heating element from excessive oxidation.
As can be seen in FIG. 1, at either end of the furnace 20 is a vestibule 46, 48. At either end of the furnace are vestibules 46, 48. The vestibules 46, 48 are counterbored to accept end blocks 60, 62 which are sized to fit the process tube 21. The process tube 21 is suspended between the end blocks 60, 62. The boats 54 containing the silicon wafer 56 are loaded into the process tube 21 for processing. The boats 54 may be slid manually or automatically into the process tube 21 or suspended within the process tube on cantilevered support arms 59 constructed of silicon carbide or ceramic and quartz.
As indicated above, the operating temperature of the furnace is generally over 1000.degree. C. The furnace cycles between temperatures of approximately 800.degree. C. when the boats are loaded into the furnace process tube and over 1000.degree. C. during full operation. Precise temperature control over the length of the furnace is critical. Also as indicated above, it is imperative that the furnaces quickly come to the operating temperature and quickly cool down after operation.
Failure of these prior furnaces 20 is due to the inability of the furnaces to control the growth or expansion of the heating element, the inability to prevent failure of the ceramic fiber insulation, the inability of the spacers to properly maintain the spacing of the individual coils of the heating element, and the combined effect of these occurrences resulting in coil sag. With coil sag, individual coils touched together and short or touch the processing tube, causing either a short to occur if the tube is made of a conductive material or causing the tube to break should the tube be made of quartz or ceramic.
Heating Element Growth:
With respect to growth of the heating elements 26, 28, 30, it is to be understood that the aluminum oxide layer formed on the exterior of the elements has a lower coefficient of expansion than the element alloy itself. As the temperature of the elements goes down, the aluminum oxide layer and the elements both contract, but of course not at the same rate. The lower coefficient of expansion of the aluminum oxide layer causes tensile stresses to form in the heating elements and compressive stresses to form in the aluminum oxide layer. Similarly, when the temperature goes up, the oxide layer and the elements both expand, but again at different rates. The lower coefficient of expansion of the aluminum oxide layer causes compressive stresses to form in the heating element and tensile stresses to form in the aluminum oxide.
These stresses cause two effects. First it is to be understood that the aluminum oxide layer has a low resistance to tensile stress. Thus as the temperature increases, the aluminum oxide layer develops cracks. The cracks in the aluminum oxide layer reduce the layers ability to retard wire elongation. Second, each time the temperature of the element exceeds 1000.degree. C., a new oxide forms. The new oxide fills the cracks in the original aluminum oxide layer, thereby looking into the heating element, the initial growth. This phenomena of aluminum oxide cracking, heating element growth and the subsequent filling in of the cracks repeats with each temperature cycle. Extreme and rapid temperature changes increase the number of fractures in the aluminum oxide layer.
The higher the operating temperature of the heating element, the greater the thermal expansion of the heating element which also further increases the cracking of the aluminum oxide layer. As the number of fractures in the oxide layer increases, the growth of the heating element accelerates. As can be understood, the growth of the heating element is a major cause of premature heating element failure in such diffusion furnaces and in particular in the high temperature, large diameter furnaces due to heating element sagging.
Insulation:
Further accelerating the failure of the diffusion furnace 20 is the failure of the insulating material. The ceramic fiber used in the insulating material which holds the spacers in place also has certain characteristics that contribute to the failure of the furnace and in particular, the failure of the heating element. First the insulation shrinks at high temperature. At 1000.degree. C., the shrinkage is approximately 0.4%, while at 1300.degree. C. the shrinkage can exceed 3.0%. Secondly, the insulation devitrifies at elevated temperatures. Devitrification means that the fibers of the ceramic insulation breakdown and become crystalline in structure. Third, the fibers loose resiliency at approximately 500.degree. C. Resiliency is the ability of the fibers to spring back after compression. Resiliency is 80% at a temperature of approximately 480.degree. C. Loss of resiliency accelerates at temperatures over 480.degree. C. and at 900.degree. C. resiliency is only about 50%.
Heating Element Failure:
As the temperature of the furnace increases, so does the growth of the heating element, and also the rate of devitrification, shrinkage and loss of resiliency in the insulation. As the coils grows, they rub against the insulation breaking the ceramic fibers into powder. The powdering of the insulation destroys its ability to retard the growth of the heating element and can additionally contaminate the furnace with the powdery material. Eventually, the combination of the coil growth and the insulation failure allows the ceramic spacers, which hold the individual coils of the heating element in place, to loosen. With degradation of the insulation and thus the ability of the insulation to maintain the position of the spacers, the individual spacers can fall out from between the individual coils allowing further growth, distortion and kinking of the heating element. The weight of the heating element itself, then can cause the element and the spacers to sag resulting in failure as indicated hereinabove.
Current spacer designs, as shown by the prior art spacers of FIGS. 2 and 3, are not satisfactorily effective in extending the life of the heating element. The individual type spacer (FIG. 3) is more effective than the comb type spacer (FIG. 2) in keeping the coil within the recesses. Once, however, the integrity of the insulation is compromised, these individual spacers can come out of alignment with respect to the adjacent spacers.
The use of more spacers could be effective in physically restraining the coil. However, the use of additional spacers adds mass around the heating element. With more mass around the heating element, the heating element becomes less responsive to the heating and cooling cycles required for semiconductor manufacture. Some prior art devices have attempted to cement the coil with respect to the spacers. This has, however, increased the temperature differential between the heating element and the portion of the chamber where the wafers are positioned. This temperature differential means that the furnace may not be able to reach appropriate temperature levels for the manufacturing operation.